The present invention relates to a drive circuit for printers and here particularly for printers of the matrix variety having needles or hammers as print impact elements.
These impact elements, needles or hammers, for matrix printers, usually are operated electromagnetically by means of solenoids. The solenoids receive brief current pulses which are timed by the printing program in general and result from a selective control of the respective impact print element pursuant to the generation of individual characters which are composed from individual dots, such dots being produced by the several impact elements. The electronic circuit included in such a printer is usually provided with a character generator which provides print or no print commands to each of the solenoid operated impact elements print or no-print. The electronic circuit includes particularly timed signal generators which determine the specific timing of printing as well as enabling signals which decide whether or not a particular solenoid is to be activated. These signal are passed through logic circuit and operate, for example, monostable multivibrators which when triggered provide pulses of fixed duration, these pulse determine the timing and duration of solenoid energization. The solenoid is immediately controlled through transistors which provide the requisite power for operating the solenoid while they themselves are controlled by the aforementioned monostable electronics.
In response to such a timed and duly amplified control pulse for a hammer or a needle, the solenoid causes the hammer or needle to be propelled forward to provide for the dot imprinting impact upon the sheet or the like to be printed on. Thereby it is feasible to operate directly electromagnetically in that the hammer or needle is in fact electromagnetically advanced and, for example, retracted after the impact by operation of a spring. Alternatively, the impact elements may normally be held back electromagnetically while the drive pulse current causes the magnetic holding field to collapse and now a spring can prevail to propel the impact element forward.
After an impact element has been advanced and retracted again, a relative shift in lateral direction is provided between the print head containing the needles or hammers and the sheet to be printed on. Depending upon the type of printer and the extent of print elements provided for, the next print step may involve just printing a different portion of the character or different portions of groups of characters, or, in the case of a strip printer, this strip may be advanced. Thus, sequential print cycles require some kind of shift between the sheet or strip printed on and the print head, hammer banks, etc.
Independent from the type of operation and generation of the overall print pattern and image, it is readily apparent that the overall print speed depends, on the one hand, on the speed with which the above mentioned shift from one print position to the next print position can be produced and additionally, the print speed depends on the repetition rate of energization of the solenoid or other electromagnetic structure operating the impact elements. In this regard it was found that the, for example, that the print head or hammer bank advance or even an incremental paper shift are the faster operations. The repetition rate of operating a print hammer or needle was found to be severely limited by the electromagnetic mode of operation and by the physical structure involved. The maximum rate attainable is usually called the limit frequency of operating such a hammer or print needle.
At the present time it is generally accepted that the needle printers have a limit frequency which does not exceed 2,000 impacts per second. However, even this number appears to be a theoretical limit only. The physical conditions for driving a print needle or print hammer usually do not permit the extension of the operating frequency up to the limit frequency. The reason for this is that the energy which is fed to the electromagnetic coil cannot be converted therein completely into kinetic energy for and in the impact element. Rather, a considerable amount and portion of that electrical energy is converted into waste heat. Aside from the loss in effectively usable energy, the printer and its operating elements may become quite warm, even hot, and transmit that heat into the immediate environment. This heat may in fact interfere with the head operation itself and it may also affect electronic circuit elements such as transistors and other semiconductor elements. Moreover, the life of the electronics when required to operate in a hot environment is reduced. Therefore, it has even been necessary in cases to provide for a forced cooling of the printer in order to remove the heat. Obviously, this is an increase in expenditure and bulkiness and renders the device more prone to break down. Moreover, maintenance of the printer is increased.